Masked photocathode with first and second metallic patterns

ABSTRACT

A masked photocathode is provided having on a radiation transmissive support a mask pattern of a material that absorbs or reflects sensitizing radiation, particularly in the ultraviolet. In one form, the mask is formed in a process that includes at least partial oxidation of a metal layer pattern. Alternatively, a mask pattern that blocks all radiation with high reflectivity may be formed by employing a metal with a subsequent insulating layer, such as of an oxide of the metal or a separately deposited insulator, for protection from the photocathode. In another form the photoemissive layer may be deposited directly on the transmissive support and have on its surface a pattern of organic material to provide the mask for electrons on the target side of the cathode as well as to prevent any radiation reflected from the target from impinging on areas from which photoemission is not desired.

United States Patent [72] Inventors [21 1 Appl. No. [22] Filed [45]Patented [7 3 Assignee Terence W. O'Keefe Pittsburgh;

Jerome R. Morris, Trafiord, both of, Pa.

[54] MASKED PHOTOCATHODE WITH FIRST AND SECOND METALLIC PATTERNS3,254,253 5/1966 Davis et al. 313/102 3,310,701 3/1967 Heimann 313/943,313,971 4/1967 Nagy 313/94X 3,368,919 2/1968 Casale etal..... 117/212X 3,443,915 5/1969 Wood et al. 29/194 X Primary Examiner-John KominskiAssistant Examiner Palmer C. Demeo Attorneys-F. Shapoe, C. L. Menzemerand G. H. Telfer ABSTRACT: A masked photocathode is provided having on aradiation transmissive support a mask pattern of a material that absorbsor reflects sensitizing radiation, particularly in the ultraviolet. Inone form, the mask is formed in a process that includes at least partialoxidation of a metal layer pattern. Alternatively, a mask pattern thatblocks all radiation with high reflectivity may be formed by employing ametal with a subsequent insulating layer, such as of an oxide of themetal or a separately deposited insulator, for protection from thephotocathode. In another form the photoemissive layer may be depositeddirectly on the transmissive support and have on its surface a patternof organic material to provide the mask for electrons on the target sideof the cathode as well as to prevent any radiation reflected from thetarget from impinging on areas from which photoemission is not desired.

MASKED PHOTOCATIIODE WITH FIRST AND SECOND METALLIC PATTERNSACKNOWLEDGEMENT OF GOVERNMENT CONTRACT The invention herein describedwas made in the course of or under a contract with the Department oftheAir Force.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention isrelated to masked photocathodes, particularly those intended forexposure to an air environment, useful in the fabrication ofmicroelectronic components.

2. Description of the Prior Art In copending application Ser. No.724,839 filed Apr. 29, 1968, by T. W. O'Keeffe and assigned to theassignee of the present invention, there were disclosed techniques forthe fabrication of masked photocathodes operable in an air environmentand employing a layer of material selectively absorbing ultravioletradiation such as a material containing titanium ions. In one form themask material was deposited on the surface of a transmissive substratethat had been etched in the desired mask pattern so that upon subsequentpolishing a planar surface with an embedded mask was achieved. It is nowconsidered preferable to avoid the polishing operation because such astep tends to produce scratches in the transmissive substrate, such asquartz, and also in the mask layer unless considerable care is taken.Scratches in either portion can produce flaws in the projected image.Minor scratches occurring in the surface of the polished quartz startingmaterial are also accentuated in the etching step.

Mask layers that selectively absorb ultraviolet may transmit otherwaveband radiation that can impinge the target and cause thermalproblems in it or in the photocathode.

An additional problem in some cases with a mask on the input side of thephotoemitter is that the photoemissive layer may not utilize all theultraviolet radiation impinging thereon with the result that some passesthrough and impinges on.the target. Such radiation may be reflected fromthe target and returned to the photoemitter with all of thephotoemissive area exposed so that spurious emission results.

SUMMARY OF THE INVENTION This invention has among its purposes andobjects the provision of improved masked air stable photocathodes andimproved methods for their fabrication, to avoid or minimize the abovementioned problems.

By improvement over the technique of the above copending application alayer of metal, such as titanium, chromium, or

aluminum, is deposited on the flat surface of a transmissive support,formed in the desired mask pattern and then at least partially oxidizedor otherwise protected by an insulating layer so that upon subsequentdeposition of a photoemissive layer it is protected in the event thephotoemissive layer is removed or replaced.

In another form of the invention, the photoemissive layer is applieddirectly to the radiation transmissive support and the mask is providedby elements of electron opaque organic material on the target side ofthe photoemitter to avoid effects of reflected radiation.

BRIEF DESCRIPTION OF THE DRAWING DESCRIPTION OF THE PREFERREDEMBODIMENTS In FIG. 1 is a radiation transmissive support 10, suitablyof fused quartz. The support 10 has a planar surface 11 on which is acontinuous metal layer 12. The metal layer may be of any of a number ofmetals, such as titanium, aluminum, and chromium, selected in accordancewith properties as will be described hereinafter.

In FIG. 2, the structure is shown after the metal layer 12 has beenprocessed into a first metallic pattern 12A, such as by conventionalphotolithographic techniques.

In FIG. 3, the metal pattern 12A has disposed over the surface of eachelement a layer 14 of a protective insulator which has been formed bychemical reaction with the material of the first metallic pattern, suchas by oxidation. The insulating layer 14 is preferably one thatselectively absorbs the radiation to which the photocathode issensitive, particularly ultraviolet radiation. The reason for this isbecause the oxidation, or other chemical reaction, is consequentlynoncritical and all of the metal may be converted. For this purpose itis preferable to form the metallic pattern of titanium, the oxide ofwhich contains titanium ions that selectively absorb ultravioletradiation. However, it may be preferable in some cases to retain aportion of the first metallic pattern 12A to reflect unabsorbedradiation, such as the visible band, so that it does not pass throughthe structure and produce adverse thermal effects in the target.

Preparation of the structure of FIG. 3 may be by first lightly oxidizingthe surface of the continuous layer 12 of FIG. 1 before forming thepattern and then partially oxidizing the remaining titanium at about400C, for about 1 to 3 hours for a titanium layer about 400 angstromsthick. If the titanium is completely oxidized the resulting oxidepattern should be about 700 angstroms or more thick.

In FIG. 4 the structure has applied over the protected metallic patternlayer a continuous layer 16 of a photoemitter. In the context of thisinvention it is preferred that the photocathode be of an air stablematerial so it may be used in demountable systems. See copendingapplication Ser. No. 753,373, filed Aug. 19, 1968, now abandoned by T.W.O- Keeffe and R.M. Handy and assigned to the assignee of this inventionas Well as the above mentioned copendin g application Ser. No. 724,839.For this purpose the photocathode is at least one number selected fromthe group consisting of palladium, platinum, barium, gold, aluminum,copper and cesium iodide.

FIG. 4 thus illustrates a completed masked photocathode not only maskedagainst radiation in those areas where it is undesired to produceemission from the photocathode but also providing a metallic reflectivesurface from which other radiation such as visible radiation would bereflected to avoid heat problems in the target. I

The structure of FIG. 2 may alternatively be processed in accordancewith the sequence illustrated in FIGS. 5 and 6.

In FIG. 5 there has been applied to the surface a continuous layer 15 ofan insulating protecting material. Since this layer is continuous it isnecessary that it be radiation transmissive to the sensitizingradiation. For this purpose it may be of quartz and deposited by knowntechniques such as rf sputtering.

In FIG. 6 the structure is completed by the deposition of a photocathodelayer 16. For some purposes the sequence illustrated in FIGS. 5 and 6may be preferred particularly in that there is a wider choice inselecting the metal. It may, for example, be of aluminum which is morehighly reflective than titanium and further minimizes the adverse heateffect previously discussed.

The photocathode in accordance with this invention may have a maskpattern of any geometrical configuration. Forms of the inventionillustrated in FIGS. 4 and 6 are suitable where an electron image is tobe made that is to be applied directly to a workpiece, such as anelectron resist coated semiconductor wafer. In certain instances it isdesired that the mask produce a plurality of essentially point sourcesof electrons that are then subjected to electrostatic and/or magneticdeflection techniques to scan patterns in one or more workpieces.

Reference should be made to copending application Ser. No. 784,551,filed Dec. 18, 1968, now Pat. No. 3,519,873, by T.W. OKeeffe andassigned to the assignee of the present invention with respect to theutilization of such arrays of point electron sources.

FIG. 7 to 9 illustrate a technique of making such an array. In FIG. 7the top surface of a substrate 10 is illustrated after there has beenformed thereon a first metallic pattern 12A that comprises a pluralityof parallel stripes that leave openings 13 therebetween of substantiallyless width than the width of the metallic stripes.

In FIG. 8 the first metallic pattern 12A has been protected by aninsulating protective coating 15 substantially as in FIG. 5; thetechnique illustrated in FIG. 3 may also be employed. Subsequent theretohas been deposited a second metallic layer formed into a pattern 22,further illustrated in FIG. 9, that also comprises a plurality ofparallel metallic stripes with openings 23 therebetween of substantiallyless width than the metallic stripes, the stripes being perpendicular tothose of the first pattern 12A. An additional protective insulator layer17 and a photocathode layer 16 are then deposited continuously as shownin FIG. 10.

The two metallic patterns 12A and 22 therefore provide openings 20, FIG.9, that can be of quite small dimensions depending on the resolutionwith which each metallic pattern may be obtained. This form of theinvention is suitable where there is available a source of electrons ina line to expose resist used in making patterns, 12A and 22. Such anelectron source is often more readily obtained and precisely controlledthan a point source.

FIG. 11 and 12 illustrate a form of the invention directed to anadditional problem. If a masked photocathode in the form of FIG. 4 or 6has only a small area that is to be blocked then the unused ultravioletradiation passing through to the target could produce considerable backscattering that would produce emission from all portions of thephotocathode. Therefore in these instances it is desirable to employ areadily formed electron mask pattern on the target side of thephotocathode material.

In FIG. II the radiation transmissive support 10 has directly thereon acontinuous layer 16 of photocathode material and a subsequent layer 32from which the mask is to be formed. The

layer 32 is of an organic material that may be selectively polymerizedor made soluble (as by light or electron bombardment) and developed in amask pattern 32A as illustrated in FIG. 11. Such a material should blockelectron emission from cathode l6. Suitable materials include members ofthe group consisting of copolymers of acrylic and methacrylic acids andesters. Such material may be formed either with or without chemicalmodification involving the introduction of polar side groups in the mainnoncross linked polymeric chain, although other organic materialsincluding hydrocarbon pump oils have been used.

Photocathodes in accordance with this invention are particularly for usewith a high intensity broadband light source such as a mercury vaporlamp for fast formation of an electron image at the cathode. FIG. 13illustrated the general arrangement oflight source 40, maskedphotocathode 42 (of the form shown in FIG. 6), and an electron resistcoated workpiece 44. The workpiece 44 may include an oxidized wafer 46of semiconductor material having on the surface toward the cathode alayer 48 of organic resist material. Further description of the use ofphotocathodes in accordance with this invention may be found byreference to the above mentioned copending application Ser. No. 753,373.

We claim as our invention:

1. A masked photocathode comprising:

a radiation transmissive support member;

a first metallic pattern on said support member;

a first insulating protective layer over at least said first metallicpattern;

a second metallic pattern coinciding only in part with said firstmetallic pattern on said first insulating protective a s c r idinsulating protective layer over at least said second metallic pattern;and

a continuous layer of photocathode material over said insulatingprotective layer.

2. The subject matter or claim 1 wherein:

said first and second metallic patterns each comprise a plurality ofparallel metallic stripes with stripelike openings therebetween ofsubstantially less width than that of said parallel metallic stripes,said first and second metallic patterns being mutually perpendicular.

2. The subject matter or claim 1 wherein: said first and second metallicpatterns each comprise a plurality of parallel metallic stripes withstripelike openings therebetween of substantially less width than thatof said parallel metallic stripes, said first and second metallicpatterns being mutually perpendicular.